Gate drivers for stacked transistor amplifiers

ABSTRACT

Various methods and circuital arrangements for biasing one or more gates of stacked transistors of an amplifier are possible where the amplifier is configured to operate in at least an active mode and a standby mode. Circuital arrangements can reduce bias circuit and stacked transistors standby current during operation in the standby mode and to reduce impedance presented to the gates of the stacked transistors during operation in the active mode while maintaining voltage compliance of the stacked transistors during both modes of operation.

CROSS REFERENCE TO RELATED APPLICATIONS—CLAIM OF PRIORITY

This application is a continuation of, and claims the benefit ofpriority under 35 USC § 120 or 35 USC § 121 of, commonly assigned andco-pending prior U.S. application Ser. No. 16/882,061, filed May 22,2020, entitled “Gate Drivers for Stacked Transistor Amplifiers”, toissue on Nov. 30, 2021 as U.S. Pat. No. 11,190,139; application Ser. No.16/882,061 is a continuation of, and claims the benefit of priorityunder 35 USC § 120 or 35 USC § 121 of, commonly assigned prior U.S.application Ser. No. 16/240,601, filed Jan. 4, 2019, entitled “GateDrivers for Stacked Transistor Amplifiers” (now U.S. Pat. No. 10,700,642issued Jun. 30, 2020); application Ser. No. 16/240,601 is a continuationof, and claims the benefit of priority under 35 USC § 120 or 35 USC §121 of, commonly assigned prior U.S. application Ser. No. 15/690,115,filed Aug. 29, 2017, “Gate Drivers for Stacked Transistor Amplifiers”(now U.S. Pat. No. 10,389,306 issued Aug. 20, 2019); application Ser.No. 15/690,115 is a divisional of, and claims the benefit of priorityunder 35 USC § 120 or 35 USC § 121 of, commonly assigned prior U.S.application Ser. No. 15/268,275, filed Sep. 16, 2016, “Gate Drivers forStacked Transistor Amplifiers” (now U.S. Pat. No. 9,843,293 issued Dec.12, 2017); the present application is related to U.S. application Ser.No. 15/268,297 entitled “Standby Voltage Condition for Fast RF AmplifierBias Recovery” filed on Sep. 16, 2016 (now U.S. Pat. No. 9,837,965issued Dec. 5, 2017), the disclosure of which is incorporated herein byreference in its entirety; the present application is also related toU.S. application Ser. No. 15/268,229 (pending) entitled “CascodeAmplifier Bias Circuits” filed on Sep. 16, 2016, the disclosure of whichis incorporated herein by reference in its entirety; the presentapplication is also related to U.S. application Ser. No. 15/268,257entitled “Body Tie Optimization for Stacked Transistor Amplifier” filedon Sep. 16, 2016 (now U.S. Pat. No. 9,882,531 issued Jan. 30, 2018), thedisclosure of which is incorporated herein by reference in its entirety.

The present application may be related to U.S. Pat. No. 7,248,120,issued on Jul. 24, 2007, entitled “Stacked Transistor Method andApparatus”, the disclosure of which is incorporated herein by referencein its entirety; the present application may be related to issued U.S.Pat. No. 9,716,477 issued Jul. 25, 2017, entitled “Bias Control forStacked Transistor Configuration”, the disclosure of which isincorporated herein by reference in its entirety; the presentapplication may also be related to issued U.S. Pat. No. 9,667,195 issuedMay 30, 2017, entitled “Amplifiers Operating in Envelope Tracking Modeor Non-Envelope Tracking Mode”, the disclosure of which is incorporatedherein by reference in its entirety; the present application may also berelated to issued U.S. Pat. No. 9,413,298 issued Aug. 9, 2016, entitled“Amplifier Dynamic Bias Adjustment for Envelope Tracking”, thedisclosure of which is incorporated herein in its entirety; the presentapplication may also be related to U.S. Pat. No. 9,219,445 entitled“Optimization Methods for Amplifiers with Variable Supply Power”, issuedDec. 22, 2015, the disclosure of which is incorporated herein byreference in its entirety; the present application may also be relatedto U.S. Pat. No. 8,487,706 B2 entitled “Stacked Linear Power Amplifierwith Capacitor Feedback and Resistor Isolation”, issued Jul. 16, 2013,the disclosure of which is incorporated herein by reference in itsentirety.

BACKGROUND 1. Field

The present application relates to amplifiers. In particular the presentapplication relates to gate biasing for amplifiers comprising stackedtransistors which can operate in an active mode during periods of RFsignal transmission, and in a standby mode during periods ofnon-transmission.

2. Description of Related Art

In recent years, stacked cascode amplifiers, which use a plurality oftransistors arranged as a stack (stacked transistors) in anamplification stage of the amplifiers, have become predominant in radiofrequency (RF) applications where high power, high voltage outputs aredesired. Due to the higher number of transistors in the stack, voltagehandling performance of the amplifier is increased, thereby allowing thehigh power, high voltage outputs. Since the stacked transistors compriseindividual low voltage transistors which can tolerate a voltagesubstantially lower than the output voltage of the amplifier, it isimportant to bias the low voltage transistors of the stack so as tomaintain operation within their tolerable voltage range. Such voltagecompliance of the low voltage transistors of the stack must bemaintained whether the amplifier operates in an active mode,transmitting a signal, or in a standby mode, not transmitting a signal.However, conflicting characteristics of a biasing circuit that providesbiasing voltages to the staked transistors may exist between operationin the active mode and in the standby mode, such as, for example, animpedance presented to the gates of the transistors of the stack duringthe active mode of operation, and a power consumed in the biasingcircuit and in the stacked transistors during the standby mode ofoperation.

SUMMARY

The various teachings according to the present disclosure describebiasing circuits for providing biasing voltages of the stackedtransistors that have different characteristics between the active modeand the standby mode, while maintaining voltage compliance of the lowvoltage transistors of the stack for safe operation in both modes ofoperation and reducing overall leakage current in the standby mode.

According to a first aspect of the present disclosure, a circuitalarrangement is presented, the circuital arrangement comprising: atransistor stack configured to operate as an amplifier, the transistorstack comprising a plurality of stacked transistors comprising an inputtransistor and an output transistor; the transistor stack configured tooperate between a first supply voltage coupled to the output transistorand a reference voltage coupled to the input transistor; a resistiveladder network comprising a plurality of series connected resistorscoupled between a second supply voltage and the reference voltage, theresistive ladder network defining gate bias voltage nodes between anytwo connected resistors of the series connected resistors; one or moreswitching impedance elements, each coupled between one or more gate biasvoltage nodes of the gate bias voltage nodes and a gate of a transistorof the plurality of stacked transistors except the input transistor,wherein the circuital arrangement is configured to operate in at least afirst mode and a second mode, wherein during operation in the firstmode, the each switching impedance element presents a first impedance tothe gate of the transistor, and during operation in the second mode, theeach switching impedance element presents a second impedance, largerthan the first impedance, to the gate of the transistor, and whereinduring operation in the first mode and in the second mode, the eachswitching impedance element presents a gate bias voltage at the one ormore gate bias voltage nodes to the gate of the transistor.

According to a second aspect of the present disclosure, a circuitalarrangement is presented, the circuital arrangement comprising: atransistor stack configured to operate as an amplifier, the transistorstack comprising a plurality of stacked transistors comprising an inputtransistor and an output transistor; the transistor stack configured tooperate between a first supply voltage coupled to the output transistorand a reference voltage coupled to the input transistor; a firstresistive ladder network comprising a plurality of series connectedresistors coupled between a second supply voltage and the referencevoltage, the resistive ladder network defining low impedance gate biasvoltage nodes between any two connected resistors of the first seriesconnected resistors; a second resistive ladder network comprising aplurality of series connected resistors coupled between the secondsupply voltage and the reference voltage, the resistive ladder networkdefining high impedance gate bias voltage nodes between any twoconnected resistors of the second series connected resistors; and one ormore switches configured to selectively couple one of the low impedancegate bias voltage nodes and the high impedance gate bias voltage nodesto gates of transistors of the plurality of stacked transistors exceptthe input transistor.

According to a third aspect of the present disclosure, a method forbiasing a transistor stack is presented, the method comprising: during afirst mode of operation of the transistor stack, coupling gates oftransistors of the stack, except an input transistor of the transistorstack, to low impedance nodes; during a second mode of operation of thetransistor stack, coupling said gates to high impedance nodes; based onthe coupling and the coupling, providing biasing voltages to said gatesduring both the first and second modes of operation according to adesired voltage distribution of a voltage across the transistor stack.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of exampleembodiments, serve to explain the principles and implementations of thedisclosure.

FIG. 1 shows a prior art embodiment of a stacked transistor amplifierwhere a resistive ladder network is used to provide biasing voltages tothe gates of the transistors of the stack.

FIG. 2 shows a prior art embodiment of a stacked transistor amplifiersimilar to the prior art embodiment depicted in FIG. 1 , where a diodeconnected transistor is used to reduce variation of the biasingvoltages, and gate capacitors are used to allow voltages at the gates ofthe transistors of the stack to vary along with an RF signal at thedrains of the transistor.

FIG. 3 shows an embodiment according to the present disclosure of aswitchable biasing circuit that can switch an impedance presented to thegates of the transistors while maintaining proper biasing of thetransistors.

FIG. 4A and FIG. 4B show exemplary embodiments of source-followertransistor circuits used in the switchable biasing circuits of thepresent disclosure.

FIG. 5 shows an exemplary embodiment according to the present disclosureof the switchable biasing circuit of FIG. 3 using the exemplarysource-follower transistor circuit of FIG. 4A.

FIG. 6 shows an impedance control element that when coupled to the priorart embodiments depicted in FIG. 1 and FIG. 2 provides functionality ofthe switchable biasing circuit of the present disclosure depicted inFIG. 3 .

FIG. 7 shows a switchable biasing circuit according to the presentdisclosure, where a plurality of impedance control elements, similar tothe one depicted in FIG. 6 , powered by different voltages, are used toeach provide a biasing voltage to a different transistor of the stack.

FIG. 8 shows a switchable biasing circuit according to the presentdisclosure where nodes of two resistive ladder networks provide biasingvoltages to transistors of the stack.

FIG. 9A shows a resistive ladder network according to an embodiment ofthe present disclosure which may be used in the switchable biasingcircuit of FIG. 8 , comprising a voltage limiter.

FIG. 9B shows an exemplary embodiment of a switch in the resistiveladder network of FIG. 9A.

FIG. 10A shows a switchable biasing circuit according to the presentdisclosure where nodes of two resistive ladder networks provide biasingvoltages to transistors of the stack.

FIG. 10B shows a switchable biasing circuit according to the presentdisclosure where nodes of two resistive ladder networks provide biasingvoltages to some transistors of the stack. A gate of a transistordirectly coupled to the input transistor of the stack can be selectivelygrounded.

FIG. 10C shows an alternative embodiment according to the presentdisclosure of the switchable biasing circuit of FIG. 5 , where biasingvoltages to the transistors of the stack can selectively be differentwhile maintaining proper biasing of the transistors.

FIG. 10D shows an alternative embodiment according to the presentdisclosure if the switchable biasing circuit of FIG. 10C, where supplyvoltage and reference voltage to the resistive ladder network aredifferent from ones provided to the stack.

FIG. 10E shows an alternative embodiment according to the presentdisclosure of the switchable biasing circuit of FIG. 7 , where biasingvoltages to the transistors of the stack can selectively be differentwhile maintaining proper biasing of the transistors, and where supplyvoltage and reference voltage to the resistive ladder network aredifferent from ones provided to the stack.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Throughout the present disclosure, embodiments and variations aredescribed for the purpose of illustrating uses and implementations ofinventive concepts of various embodiments. The illustrative descriptionshould be understood as presenting examples of the inventive concept,rather than as limiting the scope of the concept as disclosed herein.

FIG. 1 shows a simplified schematic of a prior art stacked cascode (RF)amplifier (100). By way of example and not of limitation, the stackedcascode amplifier (100) can comprise a stack of FET transistors (M1, M2,M3, M4) that include an input transistor M1, cascode transistors (M4,M3, M2), and an output transistor M4. An input RF signal, RF_(in),provided at an input terminal (120) of the amplifier (100) is routed toa gate of the input transistor, M1, and is amplified by the amplifier(100). A corresponding amplified output RF signal, RF_(out), is providedat a drain of the output transistor, M4, and routed to an outputterminal (130) of the amplifier. Bypass capacitors (125, 135) can beused to decouple low frequency (e.g., DC) biasing voltages provided tothe stack of transistors (transistor stack) from the RF_(in) andRF_(out) signals. A supply voltage, V_(CC), is provided to the drain ofthe output transistor, M4, through an inductor, L, and a referencevoltage (e.g., GND) is connected to a source of the input transistor M1.Biasing voltages (V_(G4), V_(G3), V_(G2)) at the gates of the cascodetransistors (M4, M3, M2) are provided by way of a resistive laddernetwork (R4, R3, R2, R1) coupled between the supply voltage, V_(CC), andthe reference voltage, GND. Nodes (V_(B4), V_(B3), V_(B2)) of theresistive ladder network (R4, R3, R2, R1) that connect any two resistorsof the resistive ladder network are coupled to respective gates of thecascode transistors (M4, M3, M2) via series resistors (R14, R13, R12) toprovide the biasing voltages (V_(G4), V_(G3), V_(G2)). As can be seen inFIG. 1 , the resistive ladder network (R4, R3, R2, R1) is not used toprovide a gate biasing voltage, V_(G1), of the input transistor M1.Instead, a biasing voltage to the input transistor, M1, can be providedat the node V_(B1). Various biasing circuits to generate such gatevoltage are described, for example, in the above referenced Published USApplication No. 2015/0270806, and Published US Application No. US2014/0184336 A1.

A person skilled in the art would know that FET transistors (M1, M2, M3,M4) are configured as a four-stage cascode amplifier. Teachings fromother documents, such as the above referenced U.S. Pat. No. 8,487,706B2, further describe stacked cascode amplifiers and methods to minimizeoutput signal distortion by way, for example, of biasing the variousgates of the transistors within the stack. The person skilled in the artmay use these teaching for further specifics on multi-stage stackedtransistors in a cascode configuration, where the stack of FETtransistors comprises a number of transistors different from four.

Although the amplifier (100) of FIG. 1 is shown to be powered by a fixedsupply voltage V_(CC), other configurations of the amplifier (100) wherethe supply voltage is variable can also be envisioned. In some exemplaryconfigurations, the supply voltage can be a voltage regulator, or aDC-DC converter. In further exemplary configurations, the supply voltagecan vary under control of an external control signal. In someconfigurations, the control signal can be a function of an envelopesignal of the input RF signal, RF_(in), or the output RF signal,RF_(out). Detailed description of biasing methods and apparatus forstacked transistor amplifiers operating from a variable supply voltagecan be found, for example, in the above referenced Published USApplication No. US 2014/0184336 A1, Published US Application No.2015/0270806 A1, and U.S. Pat. No. 9,219,445, the disclosures of whichare incorporated herein by reference in their entirety. A person skilledin the art would also know of configurations where the supply to theamplifier is in the form of a current source instead of the exemplaryvoltage source (e.g. V_(CC)) discussed in the present disclosure. Theteachings according to the present disclosure equally apply to suchdiverse supply configurations. The exemplary case of a fixed supplydiscussed in the present disclosure should not be considered as limitingwhat the applicant considers to be the invention.

Although N-type MOSFETs are used to describe the embodiments in thepresent disclosure, a person skilled in the art would recognize thatother types of transistors such as, for example, P-type MOSFETs andbipolar junction transistors (BJTs) can be used instead or incombination with the N-type MOSFETs. Furthermore, a person skilled inthe art will also appreciate the advantage of stacking more than twotransistors, such as three, four, five or more, provide on the voltagehandling performance of the amplifier. This can for example be achievedwhen using non bulk-Silicon technology, such as insulatedsilicon-on-insulator (SOI) or Silicon-on-Sapphire (SOS) technologies. Ingeneral, individual devices in the stack can be constructed using CMOS,silicon germanium (SiGe), gallium arsenide (GaAs), gallium nitride(GaN), bipolar transistors, or any other viable semiconductor technologyand architecture known. Additionally, different device sizes and typescan be used within the stack of devices.

The present disclosure describes methods and arrangements for biasingstacked transistor amplifiers, where the amplifiers are configured tooperate in an active mode to transmit an RF signal and in standby modewhere no signal is transmitted. Such amplifiers may be used withinmobile handsets for current communication systems (e.g. WCMDA, LTE,WiFi, etc.) wherein amplification of signals with frequency content ofabove 100 MHz and at power levels of above 50 mW is required. Suchamplifiers may also be used to transmit power at frequencies and toloads as dictated by downstream splitters, cables, or feed network(s)used in delivering cable television service to a consumer, a nextamplifier in an RF chain at a cellular base station; or a beam formingnetwork in a phased array radar system, and other. The skilled personmay find other suitable implementations for the present disclosure,targeted at lower (e.g. audio) frequency systems as well, such as audiodrivers, high bandwidth laser drivers and similar. As such, it isenvisioned that the teachings of the present disclosure will extend toamplification of signals with frequency content of below 100 MHz aswell.

With further reference to the amplifier (100) depicted in FIG. 1 , thebiasing voltages (V_(G4), V_(G3), V_(G2)) are such that each transistor(M4, M3, M2, M1) of the stack is biased according to a voltagecompliance of the transistor. In other words, a voltage across any twoterminals (e.g., gate, source, drain) of the transistor is within a safeoperating range of the transistor. As failure of a transistor can be astatistical function of applied voltages across the transistors, evenwhen such voltages are within the safe operating range, in someembodiments it may be desirable to subject the transistors of the stackto same voltage ranges so as to provide an equal life expectancy (e.g.,mean time before failure) for each transistor of the stack. Accordingly,the gate biasing voltages (V_(G4), V_(G3), V_(G2)) can be configured toevenly distribute the voltage across the transistor stack, V_(CC),amongst the stacked transistors (M4, M3, M2, M1). In other words, adrain to source voltage, V_(DS), of each transistor (M4, M3, M2, M1) ofthe stack can be made to be substantially equal to a quarter (¼) of thevoltage provided by the supply voltage V_(CC). This can be done, forexample, as described in the above referenced Published US ApplicationNo. 2015/0270806 A1, whose disclosure is incorporated herein byreference in its entirety, by biasing the gates of the transistors (M4,M3, M2) with respective biasing voltages (V_(G4), V_(G3), V_(G2)) equalto V_(CC)×¾+V_(GS), V_(CC)× 2/4+V_(GS), and V_(CC)×¼+V_(GS). In somealternative embodiments, it may be desirable to distribute the voltageacross the transistor stack, V_(CC), according to an unequaldistribution, where some of the transistors see a larger V_(DS) dropthan others, while maintaining all the V_(DS) of the transistors of thestack within their safe operating ranges. Some exemplary cases of suchalternative embodiments are described, for example, in the abovereferenced Published US Application No. 2015/0270806, whose disclosureis incorporated herein by reference in its entirety, by biasing thegates of the transistors (M4, M3, M2) with respective biasing voltages(V_(G4), V_(G3), V_(G2)) equal to V_(CC)×K₄+V_(GS), V_(CC)×K₃V_(GS), andV_(CC)×K₂V_(GS), where constants (K₄, K₃, K₂) determined the unequalvoltage distribution across the transistors of the stack.

A person skilled in the art would understand that during operation ofthe amplifier (100), an amplified RF signal at the drain of the outputtransistor (M4) can be at a voltage level substantially higher than theV_(CC) supply voltage. This means that if the gate voltage V_(G4) of theoutput transistor M4 is maintained to the biasing voltage level providedby the resistive ladder network (R4, R3, R2, R1) discussed above, andtherefore the source of M4 is maintained to, for example,V_(CC)×¾+V_(GS), then the drain to source voltage, V_(DS), of the outputtransistor M4 can be subjected to higher voltage excursions, which canbe beyond the tolerable voltage range of the transistor.

Based on the above, it can be desirable to control the stress on theindividual transistors of the stack, due to unequal voltage division ofthe voltage at the drain of the output transistor M4 across thetransistors (M4, M3, M2, M1), which may subject any one of thetransistors to a voltage beyond the tolerable voltage range of thetransistor (e.g. close to or larger than its limit breakdown voltage).This can be accomplished by configuring the gates of the transistors(M4, M3, M2) of the stack to float via insertion of a gate capacitor(C₄, C₃, C₂) as depicted in FIG. 2 . The value of the gate capacitor ischosen so to allow the gate voltage to vary along (float) with the RFsignal at the drain of the corresponding transistor, which consequentlyallows control of the voltage drop (e.g., V_(DS)) across thecorresponding transistor, thus controlling the conduction of thetransistor in accordance to the voltage at its drain, for a moreefficient operation of the transistor. Teachings about this floatingtechnique, also referred to as conduction controlling circuit, can befound in the above referenced U.S. Pat. No. 7,248,120, which isincorporated herein by reference in its entirety

As the gate capacitors (C₄, C₃, C₂) depicted in FIG. 2 allow coupling ofthe RF signal at the gates of the transistors (M4, M3, M2), suchcoupling may negatively influence operation of the biasing circuitprovided by the resistive ladder network (R4, R3, R2, R1) as variousharmonics of the RF signal, including harmonics at lower frequencies,can alter the operating bias voltages provided by the biasing circuit.As a person skilled in the art would understand, the coupled RF signal,and corresponding harmonics, at a gate of a transistor (e.g., M4, M3,M2) can generate small currents that when fed to an output impedance ofthe biasing circuit presented to the gate of the transistor, cangenerate a corresponding voltage at frequencies substantially lower thanthe frequency of the RF signal. Specifically, if the output impedance ofthe biasing circuit is large enough, such small currents can generate alarge enough low frequency voltage that adds to the biasing voltage tonegatively influence operation of the amplifier. As the output impedanceof the biasing circuit is a function of resistance values of theresistors (R4, R3, R2, R1), reducing the effects of the RF coupling inthe prior art amplifier (200) depicted in FIG. 2 is performed bychoosing such values to be smaller. In turn, such small resistancevalues of the resistors (R4, R3, R2, R1) can require higher biasingcurrents from the supply voltage V_(CC) to provide the desired gatebiasing voltages for the transistors (M4, M3, M2), as compared tochoosing higher resistance values (and being subjected to higher levelsof the RF coupling). According to some embodiments the impedance (e.g.resistance) of the biasing circuit presented to the gates of thetransistors (M4, M3, M2) is substantially of a same value, althoughother configurations are also possible where an imbalance in thepresented impedances exists.

With continued reference to the amplifier (200) of FIG. 2 , the desireto reduce the coupling of the RF signal to the basing circuit thatgenerates the gate biasing voltages of the transistors (M4, M3, M2), canincrease power dissipation in the resistive ladder network (R4, R3, R2,R1). Although such increase in power dissipation can provide advantagesduring an active mode of operation of the amplifier (reduced RF couplingdue to lower impedance), no advantages are provided during a standbymode of operation. As the amplifier (200) is not amplifying in thestandby mode, no RF signal is present in the transistor stack (M4, M3,M2, M1) and therefore no coupling effect of the RF signal to the biasingcircuit exists. However, biasing voltages, whether same or different, tothe gates of the transistors (M4, M3, M2) must be maintained during thestandby mode of operation of the amplifier (200) so as to maintainoperation of the transistors (M4, M3, M2, M1) of the stack within theirtolerable voltage ranges.

With further reference to the amplifier (200) of FIG. 2 , during thestandby mode of operation, a bias current through the stackedtransistors (M4, M3, M2, M1) may be removed (equal to a leakage current)by providing, for example, a 0 V bias voltage to the gate of the inputtransistor M1 at node V_(B1). This turns the transistor M1 in an OFFcondition where a small leakage current flows through the transistor.Such leakage current can in turn consume wasted power in the amplifierarrangement (200) of FIG. 2 . When switching from the standby mode ofoperation to the active mode of operation of the amplifier arrangement(200), the input transistor M1 is switched to its ON condition byprovision of a bias voltage to the gate of the transistor which islarger than a threshold voltage, Vth, of the transistor. Applicant ofthe present disclosure has found that such switching can cause a“glitch” in the stack (M1, M2, M3, M4) where associated transientcurrents disturb operating conditions (e.g., biasing points) of thetransistors (M4, M3, M2, M1) during a transition time between thestandby mode and the active mode (also called transient response, timeduring which the biasing points have not settled). Such disturbances,which may be associated with charging and discharging of inherentcapacitive structures of the transistors (e.g., gate-to-source and/orgate-to-drain capacitors) due to the transient currents during thetransition time, may in turn cause damage to the transistors of thestack by momentarily pushing the transistors outside their safe regionsof operation.

It follows that the teachings according to the present disclosureprovide methods and apparatus to reduce RF coupling effects to thebiasing circuit during an active mode of operation of a stackedtransistor amplifier, and reduce power dissipation in the biasingcircuit during a standby mode of operation of the stacked transistoramplifier, while providing gate biasing voltages to the gates of thestacked transistors (e.g., M4, M3, M2) in both modes of operation so asto operate the transistors within their respective safe operatingconditions. The teachings according to the present disclosure furtherprovide methods and apparatus to maintain safe operating conditions ofthe transistors of the stack during a transition time of the amplifierbetween its standby mode and active mode of operation. Finally, suchteachings further provide methods and apparatus to reduce the leakagecurrent in the stack (M4, M3, M2, M1) during the standby mode ofoperation of the amplifier (200).

As can be seen in FIG. 2 , an optional diode connected transistor M10 isadded to the resistive ladder network (R4, R3, R3, R1) which can allowvoltages at the nodes (V_(B4), V_(B3), V_(B2)) to track process relatedvariations that may affect characteristics of the transistors (M4, M3,M2, M1). By choosing the diode connected transistor M10 to have a samecharacteristics as for the transistors (M4, M3, M2, M1), process relatedvariations can equally affect current versus voltage response of thetransistors (M10, M4, M3, M2, M1) and therefore allow the voltages atthe nodes (V_(B4), V_(B3), V_(B2)) to track such process variations.Examples of such process variations include (but are not limited to)threshold voltage, mobility, oxide thickness, etc.

FIG. 3 shows a simplified schematic of a stacked cascode amplifier(300), similar to the amplifier (200) depicted in FIG. 2 , whichcomprises a switchable biasing circuit (e.g., R4, R3, R2, R1, 310, 315)according to an embodiment of the present disclosure that can switch animpedance presented to the gates of the stacked transistors (M4, M3, M2)while maintaining proper biasing of the transistors. It should be notedthat for clarity reasons, only one switchable element (310, 315associated to the gate of the transistor M3) of the switchable biasingcircuit of the present disclosure is depicted in FIG. 3 , as similarswitchable elements (310, 315) can be provided for biasing of thetransistors (M4, M2).

The switchable impedance element (310, 315) of FIG. 3 comprises animpedance conversion unit (310) that is coupled, at an input node of theimpedance conversion unit (310), to a node, V_(B3), of the resistiveladder network (R4, R3, R2, R1), and optionally, the diode connectedtransistor M10. The impedance conversion unit (310) is coupled, at anoutput node of the impedance conversion unit (310), to a first switchingnode of a switch (315). A second switching node of the switch (315) iscoupled to the node V_(B3). A common node of the switch (315) is coupledto the gate of the transistor M3 via the resistor R13. A control signal,Ctrl, selectively controls a conduction path coupled to the common nodeof the switch (315), between a conduction path including the output nodeof the impedance conversion unit (310) and a conduction path excludingsuch output node. The same control signal, Ctrl, can be used to enableand disable operation of the impedance conversion unit (310). Accordingto one exemplary embodiment, when disabled, no current is drainedthrough the impedance conversion unit (310).

In the exemplary configuration depicted in FIG. 3 , the position of theswitch (315) is such that the common node of the switch (315) couplesthe gate of the transistor M3 to the output node of the impedanceconversion unit (310), therefore presenting a voltage and an impedanceat the output node of the impedance conversion unit (310), to the gateof the transistor M3. In an alternate position (not shown) of the switch(315), the common node of the switch (315) couples the gate of thetransistor M3 to node V_(B3) of the resistive ladder network (R4, R3,R2, R1), therefore presenting a voltage and an impedance at node V_(B3)to the gate of the transistor M3.

According to an embodiment of the present disclosure, the impedanceconversion unit (310) is configured to convert an impedance of theresistive ladder network (R4, R3, R2, R1) presented at the node V_(B3)to a lower impedance at the output node of the impedance conversion unit(310), while maintaining a voltage level at said output node that issubstantially the same as the voltage at the node V_(B3) (which isconnected to the input node of 310). Accordingly, the voltage presentedto the gate of the transistor M3 at the common node of the switch (315)remains constant irrespective of the position of the switch (315), whilethe impedance presented to the gate of the transistor M3 at the commonnode of the switch (315) is selectively configured to be either theimpedance at node V_(B3), or the lower impedance at the output node ofthe impedance conversion unit (310). Alternatively, and as shown in FIG.10C and FIG. 10D (later described), the input node of (310) can becoupled to a node (e.g. V′_(B3) of FIG. 10C, later described) of theresistive ladder network different from a node (e.g. V_(B3) of FIG. 10C,later described) of the resistive ladder network coupled to the secondswitching node of the switch (315), and therefore allowing toselectively provide two different voltages at the gate of the transistorM3, one presenting a lower impedance and the other a higher impedance tothe gate of said transistor.

Based on the above, it follows that the switchable biasing circuit (R4,R3, R2, R1, 310, 315) according to an exemplary embodiment of thepresent disclosure depicted in FIG. 3 , allows maintaining a samebiasing voltage to the gate of the transistor M3 while selectivelycoupling/decoupling an impedance of the resistive ladder network (R4,R3, R2, R1) to/from said gate.

With further reference to the amplifier (300) of FIG. 3 , according toan embodiment of the present disclosure, the control signal, Ctrl, canbe a digital control signal to control operation of the amplifier (300)in one of the active mode and of the standby mode. Accordingly, foroperation of the amplifier (300) in the active mode of operation, thecontrol signal, Ctrl, can control the position of the switch (315) toconnect the output node of the impedance conversion unit (310) to theresistor R13, thereby presenting a low impedance and a desired biasvoltage to the gate of the transistor M3. Alternatively, for operationof the amplifier (300) in the standby mode of operation, the controlsignal, Ctrl, can control the position of the switch (315) to connectthe node V_(B3) to the resistor R13, thereby presenting a higherimpedance and the same desired bias voltage to the gate of thetransistor M3. A person skilled in the art would know of many ways tocontrol the cascode stack to operate in one of the active mode and ofthe standby mode. According to one exemplary embodiment, the controlsignal, Ctrl, may control a biasing circuit that generates a biasingvoltage for the gate of the input transistor M1 at the node V_(B1) toturn OFF the input transistor for operation in the standby mode. Asnoted above, the referenced Published US Application No. 2015/0270806whose disclosure is incorporated herein by reference in its entiretydescribes various biasing methods and apparatus for the input transistorM1.

Since during the active mode of operation of the amplifier (300) of FIG.3 the gate of the transistor is isolated from the node V_(B3), theimpedance at node V_(B3) may not affect coupling of an RF signal at thegate of the transistor M3. In turn this allows choosing the resistancevalues of the resistors of the resistive ladder network (R4, R3, R2, R1)to be high enough so as to reduce a standby current (power dissipationduring the standby mode in the resistive ladder network) in theresistors while providing a desired gate biasing voltage for thetransistor M3 (through voltage at the node V_(B3)). It follows that theswitchable biasing circuit (R4, R3, R2, R1, 310, 315) of the stackedamplifier (300) depicted in FIG. 3 allows maintaining of a desiredbiasing voltage at the gate of the transistor M3 during both operationmodes of the amplifier (300) while presenting a low impedance to saidgate for reduced RF coupling effects to the biasing voltages during theactive mode of operation, and reducing standby power dissipation in theresistive ladder network. Same effects can be produced via similarswitching impedance elements (310, 315) provided for biasing of thegates of the transistors (M2, M4), such as depicted, for example, inFIG. 7 later described.

With further reference to FIG. 3 , the impedance conversion unit (310)is configured to provide a high isolation between its input node,connected to node V_(B3), and its output node, connected to the switch(315). Furthermore, as described above, the impedance conversion unit(310) is configured to present a low impedance at its output node, andoutput a voltage at its output node which is equal to the voltage at itsinput node (V_(B3)). The low impedance provided at the output node ofthe conversion unit (310) further allows sinking and sourcing ofcurrents large enough to quickly charge/discharge inherent gate-to-drainand gate-to-source capacitors of the transistors (M4, M3, M2) responsiveto above mentioned “glitch” during the transition time between standbymode and active mode of operation, and therefore, allowing fast recoveryof the biasing conditions in spite of such “glitch”. In turn, thisallows for a quicker transient response of the amplifier arrangement(time it takes for the biasing conditions to settle) while maintainingthe transistors of the stack within their safe conditions of operationduring the corresponding transition time. A person skilled in the artwould know of many ways to implement such circuit. According to oneexemplary embodiment, a source-follower can be used as the impedanceconversion unit (310), as shown in FIG. 4A and FIG. 4B.

With further reference to the impedance conversion units (310A) and(310B) of FIG. 4A and FIG. 4B, a source-follower circuit can be used toprovide functionality of a buffer circuit that buffers nodes (V_(B4),V_(B3), V_(B2)) connected at the input node (410), while convertingtheir impedances at the output node (420). Transistor M40 is configuredas a source-follower, with a drain of the transistor M40 connected to asupply voltage, V_(REG), and the source of the transistor M40 connectedto a current sinking element (R40, 425) by way of a switch (415). In theexemplary embodiment of FIG. 4A, the current sinking element is aresistor (R40) whose size is chosen for a desired current through thetransistor M40 which determines an output impedance of the transistorM40, and therefore an output impedance at the output node (420).Furthermore, according to an exemplary embodiment, the transistor M40can have a low threshold voltage, substantially equal to 0 V, so as gateto source voltage drop of the transistor M40 is substantially equal to 0V (i.e., V_(GS)=0 V). During the active mode of operation of theimpedance conversion unit (310A) of FIG. 4A, the switch (415) is closedto allow flow of current through the resistor R40, and during thestandby mode of operation the switch (415) is opened to stop currentflow, and therefore power consumption through the impedance conversionunit (310A). Operation of the exemplary impedance conversion unit (310B)is similar to the operation of the element (310A) with the differencethat a current source (425) is used in lieu of the resistor R40 as ameans to sink current through the transistor M40. Due to its inherentsmaller physical size as compared to a resistor (e.g., R40), the currentsource can allow for an overall reduction in the size of the circuit(310B) when compared to the circuit (310A).

With further reference to the impedance conversion units (310A) and(310B) of FIG. 4A and FIG. 4B, the supply voltage, V_(REG), can be aregulated voltage independent from a voltage level of the supply voltageV_(CC), or alternatively can be a function of the supply voltage V_(CC),including V_(CC). A person skilled in the art would realize that avoltage level of V_(REG) must comply to a voltage compliance of thetransistor M40 so as to operate the transistor within its tolerablevoltage range. As voltages at the nodes (V_(B4), V_(B3), V_(B2)) of theresistive ladder network (R4, R3, R2, R1) can be different, depending ona node (V_(B4), V_(B3), V_(B2)) coupled to the input node (410) of theimpedance conversion unit (310A, 310B), a corresponding level of thesupply voltage V_(REG) may be different, as shown in FIG. 7 , laterdescribed.

FIG. 5 shows a simplified schematic of a stacked cascode amplifier (500)which uses the impedance conversion unit (310A) of FIG. 4A. A personskilled in the art would realize that biasing of the amplifier (500) isaccording to the switchable biasing discussed with respect to theamplifier (300) of FIG. 3 , where the impedance conversion unit (310) ofFIG. 3 is replaced by the source-follower configuration (310A) discussedwith respect to FIG. 4A. As noted above, for clarity reasons, only onesuch element (310A) is shown in FIG. 5 , as similar elements (310A),that may be powered by different supply voltages, V_(REG), may also becoupled between each of the (V_(B4), V_(B3), V_(B2)) nodes of theresistive ladder network (R4, R3, R2, R1) and corresponding gateresistors (R14, R13, R12). As previously noted, the four-stage cascodeconfiguration of the FIG. 5 is just an exemplary embodiment of thepresent disclosure and should not be conceived as limiting the scope ofwhat the applicant considers to be the invention, as the presentteachings equally apply to configurations having different number ofstages (e.g., 2, 3, 4, 5, and higher).

With further reference to the switchable biasing circuit (R4, R3, R2,R1, 310, 315) of the present disclosure depicted in FIG. 3 , theimpedance conversion unit (310) coupled to the switch (315) can beconsidered as an impedance control element (600) as depicted in FIG. 6 .According to the above description, the impedance control element (600)that has the functionality of coupling a voltage at its input node (410)to its output node (620) while selectively changing the impedance at itsoutput node under control of the control signal, Ctrl. In other words,during a first mode of operation (e.g., standby mode), a voltage at theoutput node (620) equals a voltage at the input node (410), and animpedance at the output node (620) equals the impedance at the inputnode (410). During a second mode of operation (e.g., active mode), avoltage at the output node (620) equals the voltage at the input node(410), and the impedance at the output node (620) is lower than theimpedance at the input node (410). Furthermore, during the first mode ofoperation, no current is drained by the impedance control element (600),and during the second mode of operation, an isolation between the outputnode (620) and the input node (410) is high. Given such functionality,it is well within the capabilities of a person skilled in the art todesign circuits for usage in the switchable biasing circuit of thepresent disclosure. Such circuits can use, for example, operationalamplifiers or discrete transistors to provide buffering of the voltageat the input node (410) while presenting a different impedance at theoutput node (620). In other words, the impedance conversion unit (310)of the impedance control element (600) may include anyone or acombination of transistors and operational amplifiers (with or withoutfeedback).

FIG. 7 shows a simplified schematic of a stacked cascode amplifier (700)which comprises a switchable biasing circuit (R4, R3, R2, R1, 602, 603,604) according to the present teachings. Each of the elements (602, 603,604) is according to the impedance control element (600) described inrelation to FIG. 6 . As previously described, each of the impedancecontrol elements (602, 603, 604) may be powered by a different (or same)supply voltage (V_(REG2), V_(REG3), V_(REG4)) to allow voltagecompliance of constituent electronic elements (e.g., transistors,operational amplifiers). As voltages at the nodes (V_(B4), V_(B3),V_(B2)) can follow the expression V_(B4)>V_(B3)>V_(B2), according to oneexemplary embodiment of the present disclosure the supply voltages(V_(REG2), V_(REG3), V_(REG4)) can also follow a similar expression,where V_(REG4)>V_(REG3)>V_(REG2). Operation of the stacked cascodeamplifier (700) of FIG. 7 is as described with respect to the operationof the amplifier (300) of FIG. 3 . According to an exemplary embodiment,the impedance control elements (602, 603, 604) present a same impedancevalue to the gates of the transistors (M4, M3, M2) during the activemode of operation of the amplifier (700). Other exemplary embodimentswhere the impedance control elements (602, 603, 604) present differentimpedance values to the gates of the transistors (M4, M3, M2) during theactive mode of operation of the amplifier (700) may be possible.

With further reference to the amplifier (700) depicted in FIG. 7 ,optional capacitors (C₆₄, C₆₃, C₆₂) can be used to further isolate thebiasing circuit from coupled RF signals at the gates (V_(G4), V_(G3),V_(G2)) of the transistors (M4, M3, M2). The combination of suchcapacitors with the series connected resistors (R14, R13, R12) cancreate a low pass filter whose cutoff frequency is chosen according to afrequency of operation of the RF signal amplified by the amplifier(700). Although not shown in the other figures of the presentdisclosure, similar capacitors can be used in any of the presentedembodiments.

The switchable biasing circuits according to the present disclosurediscussed above can use an impedance conversion unit (e.g., 310 of FIG.3 , FIG. 4A, FIG. 4B, FIG. 6 ) which comprises active components, suchas a transistor M40, as depicted in FIG. 4A, FIG. 4B and FIG. 5 , orother active components, such an operational amplifier, as discussedabove. An alternate switchable biasing circuit according to a furtherembodiment can use mainly passive components, such as resistors, for asimpler biasing configuration, as depicted in FIG. 8 , while providingthe same principles of operation as provided by the above discussedconfigurations, that is, to reduce RF coupling effects to the biasingcircuit during an active mode of operation of the stacked transistoramplifier, and reduce power dissipation in the biasing circuit during astandby mode of operation of the stacked transistor amplifier, whilemaintaining same gate biasing voltages to the gates of the stackedtransistors (e.g., M4, M3, M2) in both modes of operation.

With further reference to the amplifier (800) of FIG. 8 , two separateresistive ladder networks (R4, R3, R2, R1) and (R84, R83, R82, R81) areused to each provide biasing voltages to the gates of the transistors(M4, M3, M2, M1) by way of node voltages (V_(B4), V_(B3), V_(B2)) and(V′_(B4), V′_(B3), V′_(B2)). According to some exemplary embodiments ofthe present disclosure, the two resistive ladders can provide samebiasing voltages to the gates of the transistors (M4, M3, M2, M1).According to further exemplary embodiments of the present disclosure,the two resistive ladders can provide different biasing voltages to thegates of the transistors (M4, M3, M2, M1) while maintaining thetransistors within their respective safe operating conditions. Asdiscussed above, each ladder can have an optional diode connectedtransistor (e.g. M10, M81) to allow voltages at the nodes of the laddersto track process related variations of the stacked transistors (M4, M3,M2, M1). An optional switch (815) can be used to control a currentconduction path across the resistive ladder networks. It should befurther noted that, while the resistive ladder networks (R4, R3, R2, R1)and (R84, R83, R82, R81) in FIG. 8 (and other, later described) areshown connected to the same supply voltage V_(CC), other configurations,such as one depicted in FIG. 10D (later described), where a supplyvoltage to the resistive ladder networks is different than V_(CC) arealso possible.

The resistive ladder network (R4, R3, R2, R1) of FIG. 8 has beendescribed with respect to the previous figures (e.g. FIG. 3 ) and caninclude resistance values high enough to reduce a standby currentthrough the ladder. Accordingly, a switch to completely remove a currentpath through the ladder (similar in operation to switch 815) during theactive mode of operation may not be necessary as very little current isexpected to conduct through the ladder. In some exemplary embodiments,resistance values of the resistors (R4, R3, R2, R1) can be high enoughto allow conduction of a current as small as 3 μA through the ladder. Asdescribed above, during the standby mode of operation of the amplifier(800), biasing voltages to the gates of the transistors (M4, M3, M2, M1)are provides by the nodes (V_(B4), V_(B3), V_(B2)) of the resistiveladder network (R4, R3, R2, R1), where each such nodes presents a higherimpedance to the gates of said transistors. As discussed above,selection of such nodes is provided by the switches (315).

The resistive ladder network (R84, R83, R82, R81) of FIG. 8 divides avoltage (e.g. V_(CC) as shown in FIG. 8 ) across the ladder to generatevoltages at corresponding nodes (V′_(B4), V′_(B3), V′_(B2)), which canbe substantially equal to or different from the voltages at the nodes(V_(B4), V_(B3), V_(B2)), with the difference that each of the nodes(V′_(B4), V′_(B3), V′_(B2)) presents an impedance to a gate of acorresponding transistor (M4, M3, M2, M1) which is substantially lowerthan the impedance presented by a corresponding node (V_(B4), V_(B3),V_(B2)) of the resistive ladder network (R4, R3, R2, R1). As describedabove, this allows reducing coupling effects of the RF signal conductedthrough the transistors (M4, M3, M2, M1) with respect to the biasingvoltages at the nodes (V′_(B4), V′_(B3), V′_(B2)). The lower impedancepresented by the nodes (V′_(B4), V′_(B3), V′_(B2)) is provided bychoosing smaller resistance values of the resistors (R84, R83, R82,R81), which is turn allows for a substantially larger current to flowthrough the resistive ladder network (R84, R83, R82, R81) and for aquicker transient response of the amplifier arrangement (time it takesfor the biasing conditions to settle) while maintaining the transistors(M4, M3, M2, M1) of the stack within their safe conditions of operationduring a corresponding transition time. During the standby mode ofoperation of the amplifier (800) of FIG. 8 , the switch (815) removes aconduction path to the current in the ladder. In some exemplaryembodiments, resistance values of the resistors (R84, R83, R82, R81) canbe low enough to allow conduction of a current as large as 0.8 mA, orlarger, through the ladder.

FIG. 9A shows an alternative embodiment of the resistive ladder network(R84, R83, R82, R81) of FIG. 8 , where an additional transistor, M91, inseries connection between the resistors R82 and R81, can be used toforce a desired voltage at a node V′_(B2) of the resistive laddernetwork. The transistor M91 acts as a closed switch when the switch(815) is also closed, however, when the switch (815) is open (standbymode), the transistor M91 acts as a voltage limiter, for both thevoltage at the node V′_(B2) and a voltage seen by the switch (815), sothat a transistor device coupled to the node V′_(B2) (e.g. switch 315 ofFIG. 8 ) and a transistor device forming the switch (815) (e.g. M92 ofFIG. 9B) do not see the full supply voltage (Vcc) and break down. NodeV′_(B2) and the top node of the switch (815), coupled to the transistorM81, see only roughly V_(REG) because M91 with no current (since switch815 is open) has a V_(GS) of approximately 0 V, thus shieldingtransistor devices coupled to such nodes from harmful voltage and excessleakage.

FIG. 9B shows the same resistive ladder network according to FIG. 9Awhere according to an exemplary embodiment, the switch (815) isimplemented via a FET transistor M92. A person skilled in the art wouldknow of many different ways to implement the switch (815) as theexemplary embodiment depicted in FIG. 9B should not be considered aslimiting the scope of what the applicant considers to be the invention.As discussed above with reference to FIG. 9A, the transistor M91 canprotect the transistor M92 from seeing excess voltage and reduce itsleakage current when the transistor M92 is not conducting (OFF state).

As discussed above, according to some exemplary embodiments of thepresent disclosure, voltages at nodes (V_(B4), V_(B3), V_(B2)) and nodes(V′_(B4), V′_(B3), V′_(B2)) of the resistive ladder networks (R4, R3,R2, R1) and (R84, R83, R82, R81) can be different, so as to bias thegates of the transistors (M4, M3, M2) of the stack differently in thestandby mode of operation and the active mode of operation. According toan exemplary embodiment of the present disclosure, a biasing voltage toa gate of a transistor (e.g., M4, M3, M2) in the standby mode ofoperation is lower than a biasing voltage to the gate of said transistorin the active mode of operation. According to an exemplary embodiment ofthe present disclosure, the biasing voltage at a gate of each of thetransistors (M4, M3, M2) in the standby mode of operation is smaller, bya same constant voltage value, than the biasing voltage at the gate ofsaid transistors in the active mode of operation. According to anexemplary embodiment of the present disclosure, such same constantvoltage value can be approximately 0.5 V, such as for all k,|V_(BK)−V′_(BK)|=˜0.5 V. According to a further embodiment of thepresent disclosure, a biasing voltage to the gate of a transistor, M2,of the stack, directly coupled to the input transistor, is approximately0 V in the standby mode of operation. Applicant of the presentdisclosure have found that by biasing the gates of the transistors ofthe stack differently in the standby mode of operation, a reduction inthe leakage current in the stack (M4, M3, M2, M1) can be obtained. Moreparticularly, such benefits can be obtained by biasing the gate voltagesof the stack with lower biasing voltages when compared to the biasingvoltages used in the active mode.

FIG. 10A shows an exemplary configuration of an amplifier arrangement(1000A) according to an embodiment of the present disclosure, wherevoltages at nodes (V_(B4), V_(B3), V_(B2)) and nodes (V′_(B4), V′_(B3),V′_(B2)) of the resistive ladder networks (R4, R3, R2, R1) and (R84,R83, R82, R81) can be different. According to an exemplary embodiment ofthe present disclosure voltages at nodes (V_(B4), V_(B3), V_(B2)) aresmaller than voltages at corresponding nodes (V′_(B4), V′_(B3), V′_(B2))so as to allow a reduction in the leakage current though the stack whenin the standby mode. As can be seen in FIG. 10A, the optional diodeconnected transistor M10 present in the configuration depicted in FIG. 8, has been removed, so as to allow voltage at the lower node V′_(B2) ofthe resistive ladder network (R4, R3, R2, R1) be further reduced (belowthe ˜0.5 V forward bias voltage of the diode-connected transistor M10 ofFIG. 8 ). Accordingly, in one exemplary embodiment of the amplifierarrangement depicted in FIG. 10A, the voltage at the node V′_(B2) can beapproximately 0 V for a reduction in the leakage current (in some casesup to 10× reduction) of the stack (M4, M3, M2, M1).

FIG. 10B shows an alternative embodiment according to the presentdisclosure, where the where the voltage at the gate node V_(G2) of thetransistor M2 is set to 0 V by grounding such gate node via the switch(315). This allows to keep the optional diode-connected transistor M10in the resistive ladder network (R4, R3, R2, R1) while providing a 0 Vto the gate of the transistor M2 in the standby mode for a reduction inthe leakage current of the stack (M4, M3, M2, M1). By grounding the gateof the transistor M2 in the standby mode of operation, voltage divisionof the supply voltage V_(CC) may be assumed mostly by the toptransistors M3 and M4. It should be noted that since the transistors ofthe stack are designed to withstand an RF voltage at the output of thestack (drain of M4), which can be much larger than the supply voltageV_(CC), transistors M3 and M4 may still be able to assume voltagedivision of the supply voltage V_(CC) in the standby mode where no RFvoltage is present.

FIG. 10C shows an alternative embodiment according to the presentdisclosure, and similar to the embodiment described with respect to FIG.3 , FIG. 5 and FIG. 7 , where a single resistive ladder network (R4, R3,R2, R1) coupled to an impedance control unit is used to selectively biasthe gates of the transistors (M4, M3, M2) of the stack. Referencevoltages for biasing of the gates of the transistors being provided vianodes of the resistive ladder network. In the exemplary case depicted inFIG. 10C, the resistive ladder network is configured to providedifferent voltage nodes in accordance to the desired different gatebiasing voltages for the standby and active modes of operation. Forclarity reasons, FIG. 10C shows provision of the gate biasing voltagesfor one of the transistors (e.g. M3) of the stack. Accordingly, nodeV′_(B3) of the resistive ladder network, positioned closer to the supplyvoltage V_(CC), is coupled to the gate of the transistor M40 toselectively provide gate biasing voltage for the transistor M3 in theactive mode of operation. Also, node V_(B3) of the resistive laddernetwork, positioned further from the supply voltage V_(CC), is coupledto the second switching node of the switch (315) to selectively providegate biasing voltage for the transistor M3 in the standby mode ofoperation. Since node V′_(B3) is positioned closer to the supply voltageV_(CC) than node V_(B3), gate biasing voltage to the transistor M3 inthe active mode of operation (via M40) is larger than one provided inthe standby mode of operation. Further details of the functioning of thecircuit depicted in FIG. 10C can be found in the above description withrespect to FIG. 3 , FIG. 5 and FIG. 7 .

Based on the teaching of the present disclosure, a person skilled in theart may be able to find alternative circuital implementations that allowto selectively bias the transistors of the stack in the standby mode andin the active mode, including biasing with same or different biasingvoltages, while presenting a lower impedance to the gates duringoperation in the active mode, such as to allow a reduction of RFcoupling to the biasing circuit and a reduced transition time (fastertransient response of the stack). FIG. 10D and FIG. 10E show otherexemplary embodiments of the present disclosure where the resistiveladder network is provided with a supply voltage, V_(DD), different fromthe supply voltage V_(CC) provided to the stack. Such embodiments arebased on the embodiments described with respect to FIG. 3 , FIG. 5 ,FIG. 7 and FIGS. 10A-10C.

As shown in FIG. 10D, alternatively or additionally, a referencevoltage, V_(SS), to the resistive ladder network may be different fromground (GND). This can allow, for example, generating a gate biasingvoltage for M2 in the standby mode that is substantially equal to 0 V inspite of the diode-connected transistor M10 in the resistive laddernetwork. According to some exemplary embodiment of the presentdisclosure, the gate biasing voltage for M2 can even be made negative(with respect to GND) such as to bias M2, in the standby mode, to avoltage which puts its gate-to-source voltage further away from itsthreshold voltage (V_(th)) for even a more reduction in a leakagecurrent. Same can be applied to the biasing of the input transistor, M1,in the standby mode, where a negative biasing voltage can put thegate-to-source voltage of M1 further away from its threshold voltage foreven a more reduction in the leakage current. A person skilled in theart would know of other methods of reducing leakage current in the stackduring the standby mode of operation, such as, for example, increasingthe gate length of the input transistor M1, and/or change doping of theinput transistor M1 to increase the threshold voltage (Vth) of thetransistor (and therefore farther away from the reference ground).Although such methods can be used to effectively reduce the leakage,they may involve changes in process and foundry which are more involvedthan the simpler circuital changes described above.

It should be noted that although the above embodiments according to thepresent disclosure are presented with respect to a stacked transistoramplifier (e.g., 300, 500, 700 . . . ), which is shown to be powered bya fixed supply voltage V_(CC), other configurations of such stacktransistor amplifier where the supply voltage is variable can also beenvisioned. In some exemplary configurations, the supply voltage can bea voltage regulator, or a DC-DC converter. In further exemplaryconfigurations, the supply voltage can vary under control of an externalcontrol signal. In some configurations, the control signal can be afunction of an envelope signal of the input RF signal, RF_(in), or theoutput RF signal, RF_(out). Detailed description of such amplifiersoperating from a variable supply voltage can be found, for example, inthe above referenced Published US Application No. US 2014/0184336 A1,Published US Application No. 2015/0270806 A1, and U.S. Pat. No.9,219,445, the disclosures of which are incorporated herein by referencein their entirety. A person skilled in the art would also know ofconfigurations where the supply to the amplifier is in the form of acurrent source instead of the exemplary voltage source (e.g., V_(CC))discussed in the present disclosure. The teachings according to thepresent disclosure equally apply to such diverse supply configurations.The exemplary case of a fixed supply discussed in the present disclosureshould not be considered as limiting what the applicant considers to bethe invention. Furthermore, although an exemplary non-limiting case of asingle ended RF amplifier configuration is discussed in the aboveembodiments, the teachings according to the present disclosure equallyapply to other amplifier configurations using stacked transistors, suchas, for example, differential configurations. Some such configurationsare described in, for example, the above referenced Published USApplication No. 2014/0184335 A1, Published US Application No. US2014/0184336 A1, and Published US Application No. 2014/0184337 A1, whosedisclosures are incorporated herein by reference in their entirety.

The term “MOSFET” technically refers to metal-oxide-semiconductors;another synonym for MOSFET is “MISFET”, formetal-insulator-semiconductor FET. However, “MOSFET” has become a commonlabel for most types of insulated-gate FETs (“IGFETs”). Despite that, itis well known that the term “metal” in the names MOSFET and MISFET isnow often a misnomer because the previously metal gate material is nowoften a layer of polysilicon (polycrystalline silicon). Similarly, the“oxide” in the name MOSFET can be a misnomer, as different dielectricmaterials are used with the aim of obtaining strong channels withsmaller applied voltages. Accordingly, the term “MOSFET” as used hereinis not to be read as literally limited to metal-oxide-semiconductors,but instead includes IGFETs in general.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of specifications. Unless otherwise noted above, selection ofsuitable component values is a matter of design choice and variousembodiments of the invention may be implemented in any suitable ICtechnology (including but not limited to MOSFET and IGFET structures),or in hybrid or discrete circuit forms. Integrated circuit embodimentsmay be fabricated using any suitable substrates and processes, includingbut not limited to standard bulk silicon, silicon-on-insulator (SOI),silicon-on-sapphire (SOS), GaN HEMT, GaAs pHEMT, and MESFETtechnologies. However, the inventive concepts described above areparticularly useful with an SOI-based fabrication process (includingSOS), and with fabrication processes having similar characteristics.Fabrication in CMOS on SOI or SOS enables low power consumption, theability to withstand high power signals during operation due to FETstacking, good linearity, and high frequency operation (in excess ofabout 10 GHz, and particularly above about 20 GHz). Monolithic ICimplementation is particularly useful since parasitic capacitancesgenerally can be kept low (or at a minimum, kept uniform across allunits, permitting them to be compensated) by careful design.

Voltage levels may be adjusted or voltage and/or logic signal polaritiesreversed depending on a particular specification and/or implementingtechnology (e.g., NMOS, PMOS, or CMOS, and enhancement mode or depletionmode transistor devices). Component voltage, current, and power handlingcapabilities may be adapted as needed, for example, by adjusting devicesizes, serially “stacking” components (particularly FETs) to withstandgreater voltages, and/or using multiple components in parallel to handlegreater currents. Additional circuit components may be added to enhancethe capabilities of the disclosed circuits and/or to provide additionalfunctions without significantly altering the functionality of thedisclosed circuits.

The examples set forth above are provided to give those of ordinaryskill in the art a complete disclosure and description of how to makeand use the embodiments of the gate drivers for stacked transistoramplifiers of the disclosure, and are not intended to limit the scope ofwhat the applicant considers to be the invention. Such embodiments maybe, for example, used within mobile handsets for current communicationsystems (e.g. WCMDA, LTE, WiFi, etc.) wherein amplification of signalswith frequency content of above 100 MHz and at power levels of above 50mW may be required. The skilled person may find other suitableimplementations of the presented embodiments.

Modifications of the above-described modes for carrying out the methodsand systems herein disclosed that are obvious to persons of skill in theart are intended to be within the scope of the following claims. Allpatents and publications mentioned in the specification are indicativeof the levels of skill of those skilled in the art to which thedisclosure pertains. All references cited in this disclosure areincorporated by reference to the same extent as if each reference hadbeen incorporated by reference in its entirety individually.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications can bemade without departing from the spirit and scope of the presentdisclosure. Accordingly, other embodiments are within the scope of thefollowing claims.

The invention claimed is:
 1. A method for biasing a transistor stack foroperation according to at least a first mode and a second mode, themethod comprising: during the first mode of operation, providing biasingvoltages to gates of the transistors of the stack, except an inputtransistor, via high impedance nodes of a first resistive laddernetwork; and during the second mode of operation, providing biasingvoltages to said gates via low impedance nodes of a second resistiveladder network; wherein a ratio of a current through the secondresistive ladder network during the second mode of operation and acurrent through the first resistive ladder network during the first modeof operation is equal to, or larger than,
 100. 2. The method accordingto claim 1, further comprising: based on the providing, coupling duringthe first mode of operation a respective first biasing voltage to eachof said gates; and coupling during the second mode of operation arespective second biasing voltage to each of said gates that issubstantially equal to the respective first biasing voltage.
 3. Themethod according to claim 1, further comprising: based on the providing,coupling during the first mode of operation a respective first biasingvoltage to each of said gates; and coupling during the second mode ofoperation a respective second biasing voltage to each of said gates,wherein for at least one gate of said gates, the respective firstbiasing voltage is different from the respective second biasing voltage.4. The method according to claim 1, wherein: the first mode of operationis a standby mode, and the current through the first resistive laddernetwork during the first mode of operation is equal to, or less than, 3μA.
 5. The method according to claim 4, wherein: the second mode ofoperation is an active mode, and the current through the secondresistive ladder network during the second mode of operation is as largeas 0.8 mA.
 6. The method according to claim 1, wherein: the highimpedance nodes are provided via nodes of the first resistive laddernetwork comprising a plurality of series connected resistors withresistance values according to desired impedance values of the highimpedance nodes.
 7. The method according to claim 1, wherein: the lowimpedance nodes are provided via nodes of the second resistive laddernetwork comprising a plurality of series connected resistors withresistance values according to desired impedance values of the lowimpedance nodes.
 8. The method according to claim 1, wherein: thetransistor stack comprises one or more gate capacitors each connectedbetween a gate of a transistor of the transistor stack, except an inputtransistor of the transistor stack, and a reference voltage.
 9. Themethod according to claim 8, wherein each said one more gate capacitoris configured to allow a gate voltage at the gate of the transistor tovary along with a radio frequency (RF) voltage at a drain of thetransistor.
 10. The method according to claim 9, wherein the one or moregate capacitors are configured to substantially equalize an output RFvoltage at a drain of an output transistor of the transistor stackacross a plurality of transistors of the transistor stack.
 11. Acircuital arrangement comprising: a transistor stack configured tooperate as an amplifier, the transistor stack comprising a plurality ofstacked transistors comprising an input transistor and an outputtransistor; and a biasing circuit coupled to one or more gates of theplurality of stacked transistors, the biasing circuit comprising aplurality of resistors, wherein the plurality of resistors are arrangedaccording to a first plurality of series connected resistors that defineone or more high impedance nodes of a first resistive ladder network anda second plurality of series connected resistors that define or more lowimpedance nodes of a second resistive ladder network, wherein thecircuital arrangement is configured to operate in at least a first modeand a second mode of operation, wherein during the first mode ofoperation, the one or more high impedance nodes of the biasing circuitare coupled to the one or more gates to provide respective biasingvoltages, wherein during the second mode of operation, the one or morelow impedance nodes of the biasing circuit are coupled to the one ormore gates to provide respective biasing voltages, and wherein a ratioof a current through the second resistive ladder network during thesecond mode of operation to a current through the first resistive laddernetwork during the first mode of operation is equal to, or larger than,100.
 12. The circuital arrangement according to claim 11, wherein: foreach of the one or more gates, the respective biasing voltage providedduring the first mode of operation is substantially equal to therespective biasing voltage provided during the first mode of operation.13. The circuital arrangement according to claim 11, wherein: for atleast one of the one or more gates, the respective biasing voltageprovided during the first mode of operation is different from therespective biasing voltage provided during the first mode of operation.14. The circuital arrangement according to claim 11, wherein the one ormore low impedance nodes are coupled to the one or more gates viarespective one or more switches.
 15. The circuital arrangement accordingto claim 11, wherein the one or more high impedance nodes are coupled tothe one or more gates via respective one or more switches.
 16. Thecircuital arrangement according to claim 11, wherein: the first mode ofoperation is a standby mode, and the first plurality of series connectedresistors are configured to conduct a current through the firstresistive ladder that is equal to, or less than, 3 μA.
 17. The circuitalarrangement according to claim 16, wherein: the second mode of operationis an active mode, the second plurality of series connected resistorsare configured to conduct a current through the second resistive ladderthat is as large as 0.8 mA.
 18. The circuital arrangement according toclaim 11, wherein: the transistor stack comprises one or more gatecapacitors each connected between a gate of a transistor of thetransistor stack except an input transistor of the transistor stack, anda reference voltage.
 19. The circuital arrangement according to claim18, wherein each said one or more gate capacitor is configured to allowa gate voltage at the gate of the transistor to vary along with a radiofrequency (RF) voltage at a drain of the transistor.
 20. The circuitalarrangement according to claim 19, wherein the one or more gatecapacitors are configured to substantially equalize an output RF voltageat a drain of an output transistor of the transistor stack across aplurality of transistors of the transistor stack.
 21. The circuitalarrangement according to claim 11, wherein the circuital arrangement ismonolithically integrated using a fabrication technology comprising oneof: a) silicon-on-insulator (SOI) technology, and b) silicon-on-sapphiretechnology (SOS).
 22. An electronic module comprising the circuitalarrangement of claim 11.